The present invention relates to steam turbines, such as high pressure steam turbines used in nuclear power plants, and specifically to a means for diminishing exhaust pipe erosion, as in the cross-under piping that connects the steam turbine exhaust hood and the moisture separator reheater.
The wet steam conditions associated with a nuclear steam turbine cycle have been observed to cause significant erosion/corrosion of cycle steam piping and components between the high pressure turbine exhaust and the moisture separator reheater.
The pattern, location and extent of cross-under piping erosion is a function of piping size, material and layout configuration, turbine exhaust conditions and plant load cycle. However, as a general rule, a base-loaded plant having carbon steel cross-under piping with typical nuclear high pressure turbine exhaust conditions of 12 percent moisture and 200 psia will experience, within 3 to 5 years after initial startup, erosion damage levels that require weld repair to restore minimum wall thickness. Such weld repairs are expensive and time-consuming to effect, and often result in extending planned outages. Occasionally cross-under piping erosion is the cause of an unscheduled outage.
In any event, weld repair of erosion/corrosion in cross-under piping is a very expensive proposition and the alternative approach of complete replacement of the eroded piping is even more expensive considering the time and logistics involved in such an undertaking.
Piping erosion is caused by moisture droplets impacting on the piping wall. The larger the droplet and the higher its velocity of impact, the greater the potential for mechanically removing metal from the piping wall.
Resistance to erosion is a function of the piping material's metallurgy. The carbon steel material generally favored for larger central station steam systems has an excellent service record under conventional fossil-fired steam cycle conditions, but have proven to be susceptible to erosion in nuclear reactor steam cycles. The use of more erosion resistant materials such as austenitic stainless steels, Inconel or carbon steels containing chrome or nickel are expensive alternatives.
Therefore, the incorporation of a device that could eliminate, reduce or control erosion in cross-over piping is certainly economically justifiable considering the cost of extended plant outages (especially unscheduled outages), weld repair costs and expensive alternative materials.
It is believed that most of the moisture droplets entrained in the steam leaving the high pressure turbine blading have an average diameter of less than 10 .mu.m. The remaining twenty percent, or so, of the moisture is typified by droplets ranging from 100 .mu.m to 200 .mu.m or larger.
As described in U.S. Pat. No. 4,527,396, issued to George J. Silvestri, Jr., one of the present inventors, which is assigned to the assignee of the present invention and the contents of which are incorporated herein, by virtue of their geometry, nuclear steam turbine exhaust casings create vortices in the exiting wet steam. Such vortices have been observed in curved piping, where they are known as secondary flow patterns, as illustrated in FIGS. 1 to 5 of the aforementioned patent and described in the description relevant thereto. Thus, nuclear turbine exhaust casings, by creating vortices in the two phase flow, generate a centrifugal force field causing it to function as a centrifugal separator by forcing the heavier (bigger) water droplets to migrate, or drift, through the gas phase (steam) and be deposited on the exhaust casing wall. The extent of separation depends on the steam flow (velocity), exhaust casing geometry (primarily radius of curvature), and steam condition (pressure, temperature, quality). It has been calculated by considering the resulting centrifugal force and the resisting drag force under typical exhaust steam conditions that the relative velocity of moisture droplets 50 .mu.m or bigger with respect to the steam will result in trajectories such that 20 to 30 percent of the total moisture present at the exit of the last blade row should be deposited on the exhaust casing walls. Therefore, considering the aforementioned droplet population distribution, most of the moisture droplets above 50 m in size must have been separated out and now appear as a water film on the exhaust casing walls. Hence, by trapping this film of water, the large, erosion causing droplets can substantially be removed, thus favorably altering the erosion potential of the steam exiting the high pressure turbine. Left alone, the water film on the casing walls becomes re-entrained into the steam flow at the juncture of the outlet nozzle and the exhaust casing proper, with the water film sheet being shattered into large droplets at this intersection. It is postulated that at steady state conditions, re-entrainment of this water film produces a definitive droplet size distribution and pattern which in turn leads to the observed distinctive erosion patterns downstream of the exhaust.
In short, the turbine exhaust casing provides separation of the erosion-causing fraction of the moisture, depositing these droplets as a film on the exhaust casing wall. By arranging to remove this film before it can be reentrained into the high pressure turbine exhaust steam as it passes into the outlet nozzle, cross-under piping erosion can be substantially curtailed if not altogether eliminated. Moisture pre-separators using this concept are referred to as "film-entrapment" type pre-separators.
The theory and principles of film entrapment pre-separators have been successfully demonstrated. The preseparator system for a steam turbine exhaust, described in U.S. Pat. No. 4,673,426 assigned to the assignee of the present invention, for example, was installed for tests in May-June 1984, with provisions for in-service performance testing using chemical tracer techniques. Subsequent testing in the September-October 1984 period revealed the target level of 20 percent of the moisture was being removed. However, there is ample evidence the pre-separator could likely be removing more than 20 percent, since the drains and drain collection plumbing were connected to existing plant vents and drains so as to promote the likelihood of causing the separated moisture to flash, thus reducing the effectiveness of the pre-separator. Further, the arrangement of the test injection and sampling locations did not assure complete and uniform mixing of the tracer, nor was a correction applied for flashing of separated water in the drain lines. Nevertheless, even though the tracer mixing and collected water flashing problems would tend to reduce the calculated system effectiveness, the pre-separator removed the targeted goal of 20 percent total entrained water. Equally interesting and important, the test results showed a pronounced difference in individual drain line flows, a not unexpected phenomenon, considering the existence of local vortices superimposed on the general curved path flow of the steam-water mixture in the turbine exhaust casing.
This completely in-turbine pre-separator has given no evidence of increased exhaust steam pressure loss as determined per heat rate tests, thus meeting one of the design goals.
In another installation, the in-turbine preseparator of said copending application was applied, except the pre-separator was built into a transition piping section at the base of the turbine which converted an obround turbine exhaust to the round cross-under piping geometry. This allowed the separated moisture collection "pocket" to be increased over that of the previously described system and, consequently, the use of fewer drain lines to transport the collected moisture to existing drain collection tanks. This larger collection pocket provided ample hold-up volume for generating the pressure head necessary to force the water into the drain lines, without the concern of overflowing the pre-separator pocket. Thus the residence time in the pre-separator collection pocket at that installation was increased over that available at the previous installation without causing an increase in cycle steam pressure drop due to narrowing of the cross-under piping geometry. Although test results are not definitive and the test procedure is not precise, the utility has reported 90 percent water removed. This figure is probably optimistic; however, it is abundantly clear, based on these two installations, that a film entrapment moisture pre-separator theory and practice is based on sound principles.